High-carbon steel suitable for super high tensile strength hard drawn steel wire

ABSTRACT

A high-carbon steel suitable for super high tensile strength steel wire having excellent ductility and heat treating ability, comprising 0.75-1.0% of carbon, not more than 0.5% of silicon, 0.1 to 1.0% of manganese, 0.5 to 5.0% of chromium, one or more of 1 to 4% of cobalt, 0.01 to 0.2% of niobium, 0.001 to 0.05% of titanium, 0.01 to 0.3% of vanadium and not more than 0.1% of aluminum with the balance being iron and unavoidable impurities.

United States Patent [1 1 Nagumo et al.

[ Sept. 23, 1975 HIGH-CARBON STEEL SUITABLE FOR SUPER HIGH TENSILE STRENGTH HARD DRAWN STEEL WIRE Inventors: Michihiko Nagumo, Tokyo;

Shigehiro Yamaguchi, Fujisawa; Toshihiko Takahashi, Kawasaki, all of Japan Assignee: Nippon Steel Corporation, Japan Filed: Dec. 6, 1972 Appl. No.: 312,727

Foreign Application Priority Data Dec. 6. 1971 Japan 46-97840 U.S. Cl 75/126 D; 75/126 E; 75/126 F; 75/126 H; 148/36 Int. Cl. C21D 9/52; C22C 38/26; C22C 38/28; C22C 38/30 Field of Search 75/126 H, 126 F, 126 E, 75/126 D; 29/193, 191.6; 148/36 References Cited UNITED STATES PATENTS 6/1914 Beckel 75/126 H 1.449.789 3/1923 Smith 75/126 H 1.678.001 7/1928 2.983.601 5/1961 3.507.711 4/1970 3.647.571 3/1972 Okamoto ct a1. 148/123 OTHER PUBLICATIONS Tool Steels" American Society for Metals. published 1962, pp. 226-227.

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Arthur J. Steiner Attorney, Agent, or FirmToren, McGeady and Stanger ABSTRACT 1 Claim, No Drawings HIGH-CARBON STEEL SUITABLE FOR SUPER HIGH TENSILE STRENGTH HARD DRAWN STEEL WIRE The present invention relates a high-carbon steel suitable for super high tensile strength hard drawn steel wire having excellent ductility and heat treating ability, and particularly provides a steel suitable for hard steel wire having tensile strength of more than 180 kg/mm Convcntionally, as steel wire strands for bridge cables for a suspension bridge for example, hard drawn steel wires of about 5 mm diameter, zinc coated after cold drawing, have been used, and usually a piano steel wire grade .118 Grade No. 2 Z is used for this purpose.

Rolled wire rod of about mm diameter is subjected to patenting and wire drawing to obtain wire products having tensile strength of about 170 kg/mm Recently, construction plans have been increasingly made for longer and larger bridges, and higher tensile strength of steel wire has been demanded for reducing the weight of bridges and also larger diameter of steel wire has been sought for in order to simplify the bridge construction work.

It is conventionally known that the strength of steel wire depends on the fineness of pearlitc structure of the steel wire after patenting and on the degree of reduction of area during the cold wire drawing. Therefore, increase of strength of the wire material after patenting is effective for increase of strength of hard drawn steel wire product, and for this purpose it is necessary to refine the pearlite structure.

The present inventors have made fundamental studies on functions of alloying elements for the refinement of pearlite, and have found that chromium is effective to shorten the interlamella spacing in the pearlite struc ture through the increase of transformation energy.

As clarified by the present inventors, the mechanism of the pearlite refinement by chromium is not based on the lowering of transformation temperature due to the increased hardenability as conventionally considered. In this point, an increased content of chromium is disadvantageous because it increases the lower limit of the transformation temperature of the pearlite. However, it is true that the chromium addition increases hardenability of the steel in a sense that it shifts the continuous cooling transformation curve toward the longer time side. This phenomenon causes a problem when importance is given on production efficiency because it also shifts the finishing time of the pearlite transformation toward the longer side so that longer time is required for immersion in the lead bath. Another problem is that a higher austenitizing temperature required for the dccomposition of chromium containing carbides results in the coarsening of the austenite grains so that the duetility of the transformed pearlite is reduced.

Based on the above discoveries and knowledges, the present inventors have made various studies on the effects of alloying elements to find a way to reduce the hardenability of chromium containing steels on a thought completely contrast to the conventional thought. As the result, it has been found that addition of cobalt niobium vanadium, titanium, etc. is effective for the purpose. Namely, cobalt is effective to promote the pcarlitc transformation. Thus it has been found that addition of cobalt alone in the conventional steel deteriorates the hardenability, but addition of cobalt in combination with chromium is very useful in that it compensates excessive hardenability due to chromium addition. It is also known hardenability of the steel generally depends largely on the austenite grain size. Refinement of the austenite grains in the conventional steel is disadvantageous because it deteriorates the hardenability. However, it has been found that similar effects can be obtained in promoting the transformation of a chromium-containing steel by the addition of cobalt in combination with chromium. This gives a large advantage that the time required by the patenting treatment is reduced. Refinement of the austenite grains is also found to be effective in improving the ductility of the transformed pearlite. Improvement of the ductility is getting more important for higher tensile strength steels. Addition of Nb, V, Ti and/or Al is found to be effective for this purpose.

The present invention will be described in details hereinunder.

The high-carbon steel according to the present invention comprises 0.75 to 1.0% of carbon, not more than 0.5% of silicon, 0.1 to 1.0% of manganese, 0.5% to 5.0% preferably 1.0 to 5.0% of chromium, and one or more of 1.0 to 4.0% of cobalt, 0.01 to 0.2% of niobium, 0.01 to 0.3% of vanadium, 0.001 to 0.05% of titanium, not more than 0.10% of aluminium, with the balance being iron and unavoidable impurities.

Reasons for limitations of individual elements as above in the present invention are as follows.

Carbon is necessary to be present in an amount of 0.75% at least to increase strength, but excessive carbon causes proeutectoid cementite to precipitate in the grain boundary to lower drawability and toughness. Thus, the upper limit of carbon is defined at 1.0%. Silicon is necessary for deoxidization of the steel, but is undesirable for drawability and thus limited to not more than 0.5%. I

At least 0.1% manganese is necessary for deoxidization and adjustment, but with more than 1% of manganese, a longer time is required for the completion of transformation and the interlammella spacing in the pearlite is expanded. Thus, the upper limit of manganese is defined at 1.0%. Chromium is most important for refining the pearlite and for this effect more than 0.5% preferably more than 1.0% of chromium is necessary. But if chromium is contained in an excessive amount, a very long time is required for the completion of transformation, and thus chromium is limited to 5% as its upper limit. Cobalt is added in order to prevent the delay in the completion of transformation due to the addition of chromium and manganese. However, less than 1% of cobalt is not effective to promote the transformation while more than 5% of cobalt forms lump carbides harmful to drawability. Thus, the upper limit of cobalt is defined at 5%.

Niobium, titanium, and vanadium are added in single or in combination in order to form fine carbonitrides to refine the austenite grains and lower the hardenability. For this effect, niobium is defined from 0.01 to 0.2%, titanium is defined from 0.001 to 0.05% and vanadium is defined from 0.01 to 0.3%.

Aluminum also acts for grain refinement by its nitride and is added in an amount enough for deoxidization, and thus limited to not more than 0.1%.

An example of the present invention will be set forth hereinunder to clarify the advantages of the present invcntion.

;Table 1 shows the chemical composition of the present inventive steel and Table 2 shows'the starting and finishing time of the transformation and mechanical properties under the patented condition of the present inventive steel.

Table l Chemical Compositions (weight /z) Steel C Si Mn Cr Co Al Ti others Grade A 0.84 0.24 0.51 1.02 0.047 0.005 B 0.83 0.25 0.50 0.07 2.05 0.040 0.006 c 0.34 0.27 0.51 1.05 0.035 0.005 Nb 0.04 D 0.85 0.25 0.50 1.02 0.041 0.04 E 0.85 0.25 0.49 1.00 0.045 0.005 v Table 2 the reduction of area which is indicative of the drawability, the present inventive steels show such excellent Transformation Characteristics values as about 50% of that of the conventional steel. and Mechamca' Pmpemes In this way, the addition of chromium in combination Trans- Trans Rcduc. with cobalt, niobium, titanium, vanadium, etc., elimiseel f- Y'eld nates the defect of the conventional steel that a long Grade manon matlon Strength Strength of I I Starting Finishing at 02% 2, Area tlme is required for the patenting treatment and gives Time Time remarkable advantage that super high tensile strength (sec.) (see) and improved workability of the steel wire can be ob- A 18.5 83 118.0 l50.8 43.2 tamed- B 5.3 35 124.6 l56.7 46.5 C 10.5 50 122.0 153.0 47.2 D 12.0 60 117.2 149.5 43.5 what clawed r E 13.0 65 119.5 15 44 1. A super high strength steel wire having excellent heat treating ability consisting essentially of 0.75 -l .O% Conditions of patenting of carbon, not more than 0.5% of SlllCOn, 0.l to 1.0% 1. Wire Diameter 15 mm of manganese, 1.0 to 5.0% of chrom1um, and a metal 2. Austenization at 950C for 5 minutes 3- Lwd Bath Temperature 580C selected from the group consistlng of l to 4% of cobalt,

As understood from the results, the addition of cobalt, niobium, titanium, vanadium, etc. shows remarkable effects in promoting the pearlite transformation 0.01 to 0.2% of niobium, 0.001 to 0.0 titanium, 0.01 to 0.3% of vanadium and not more than 0.1% of aluminum with the balance being iron and unavoidable im- 4O purities. 

1. A SUPER HIGH STRENGHT STEEL WIRE HAVING EXCELLENT HEAT TREATING ABILITY CONSISTING ESSENTIALLY OF 0.75 -1.0% OF CARBON NOT MORE THAN 0.5% OF SILICON, 0.1 TO 1.0% OF MANGANESE, 1.0 TO 5.0% OF CHROMIUM, AND A METAL SELECTED FROM THE GRUP CONSISTING OF 1 TO 4% OF COBALT, 0.01 TO 0.2% OF NIOBIUM, 0.001 TO 0.0 TITANIUM, 0.01 TO 0.3% OF VANADIUM AND NOT MORE THAN 0.1% OF ALUMINUM WITH THE BALANCE BEING IRON AND UNAVOIDABLE IMPURITIES. 